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arXiv:1911.00553 · Applied Physics Letters

A tunable high-Q millimeter wave cavity for hybrid circuit and cavity QED experiments

Aziza Suleymanzade, Alexander Anferov, Mark Stone, Ravi Naik, Andrew Oriani, Jonathan Simon, David Schuster

Published Mar 9, 2020• 24 citations
arXivHTMLPDF

Abstract

The millimeter wave (mm-wave) frequency band provides exciting prospects for quantum science and devices since many high-fidelity quantum emitters, including Rydberg atoms, molecules, and silicon vacancies, exhibit resonances near 100 GHz. High-Q resonators at these frequencies would give access to strong interactions between emitters and single photons, leading to rich and unexplored quantum phenomena at temperatures above 1 K. We report a 3D mm-wave cavity with a measured single-photon internal quality factor of 3×107 and mode volume of 0.14×λ3 at 98.2 GHz, sufficient to reach strong coupling in a Rydberg cavity quantum electrodynamics system. An in situ piezotunability of 18 MHz facilitates coupling to specific atomic transitions. Our unique, seamless, and optically accessible resonator design is enabled by the realization that intersections of 3D waveguides support tightly confined bound states below the waveguide cutoff frequency. Harnessing the features of our cavity design, we realize a hybrid mm-wave and optical cavity, designed for interconversion and entanglement of mm-wave and optical photons using Rydberg atoms.

DOI
10.1063/1.5137900

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The millimeter wave (mm-wave) frequency band provides exciting prospects for quantum science and devices since many high-fidelity quantum emitters, including Rydberg atoms, molecules, and silicon vacancies, exhibit resonances near 100 GHz. High-Q resonators at these frequencies would give access to strong interactions between emitters and single photons, leading to rich and unexplored quantum phenomena at temperatures above 1 K. We report a 3D mm-wave cavity with a measured single-photon internal quality factor of 3×107 and mode volume of 0.14×λ3 at 98.2 GHz, sufficient to reach strong coupling in a Rydberg cavity quantum electrodynamics system. An in situ piezotunability of 18 MHz facilitates coupling to specific atomic transitions. Our unique, seamless, and optically accessible resonator design is enabled by the realization that intersections of 3D waveguides support tightly confined bound states below the waveguide cutoff frequency. Harnessing the features of our cavity design, we realize a hybrid mm-wave and optical cavity, designed for interconversion and entanglement of mm-wave and optical photons using Rydberg atoms.
Cavity quantum electrodynamics (QED) is a powerful tool in quantum science, enabling preparation of nonclassical states of light and scalable entanglement of many atoms coupled to a single field mode. While the most coherent atom-photon interactions have been achieved using superconducting millimeter-wave cavities coupled to Rydberg atoms, these platforms so far lack the optical access required for trapping and addressing individual atomic qubits. We present a millimeter-wave Fabry-Pérot cavity with finesse 5.8 ( 1 ) × 10 7 at a temperature of 1 K providing generous transverse optical access (numerical aperture 0.56). Conflicting goals of strong atom-photon coupling and optical access motivate a near-confocal geometry. Close to confocality, however, postparaxial corrections to the cavity spectrum introduce unexpected degeneracies between transverse modes, leading to excess cavity loss. Modeling these corrections allows for tuning the cavity geometry to evade this loss, producing a high finesse that will enable cavity QED experiments with trapped atoms deep in the strong coupling regime.
Millimeter waves are emerging as an enabling technology for connecting and enhancing different quantum platforms such as Rydberg atoms, optomechanics, and superconducting qubits. In this work, we explore the interaction between millimeter wave photons and conventional transmon qubits, specifically for qubit readout. We study a circuit quantum electrodynamic (cQED) system consisting of a millimeter wave cavity at ωr=2π×34.7ω_r = 2π\times 34.7ωr​=2π×34.7 GHz and a transmon qubit at ωq=2π×3.1ω_q = 2π\times 3.1ωq​=2π×3.1 GHz coupled at rate g=2π×1.3g = 2π\times 1.3g=2π×1.3. With such a large detuning between cavity and qubit, ωr/ωq>10ω_r/ω_q > 10ωr​/ωq​>10, we are able to suppress drive induced unwanted resonant state transitions, enabling strong drives for qubit readout. We measure no resonant state transitions even when driving with more than 100100100 photons and read out the qubit state with more than 100100100 photons to achieve a measurement fidelity greater than 99%99\%99% without the aid of a quantum limited amplifier.
A millimeter-wave resonator is produced by drilling a plurality of holes into a piece of metal. Each hole forms an evanescent tube having a lowest cutoff frequency. The holes spatially intersect to form a seamless three-dimensional cavity whose fundamental cavity mode has a resonant frequency that is less than the cutoff frequencies of all the evanescent tubes. Below cutoff, the fundamental cavity mode does not couple to the waveguide modes, and therefore has a high internal Q. Millimeter waves can be coupled into any of the tubes to excite an evanescent mode that couples to the fundamental cavity mode. The tubes also provide spatial and optical access for transporting atoms into the cavity, where they can be trapped while spatially overlapping the fundamental cavity mode. The piece of metal may be superconducting, allowing the resonator to be used in a cryogenic environment for quantum computing and information processing.
A millimeter-wave resonator is produced by drilling a plurality of holes into a piece of metal. Each hole forms an evanescent tube having a lowest cutoff frequency. The holes spatially intersect to form a seamless three-dimensional cavity whose fundamental cavity mode has a resonant frequency that is less than the cutoff frequencies of all the evanescent tubes. Below cutoff, the fundamental cavity mode does not couple to the waveguide modes, and therefore has a high internal Q. Millimeter waves can be coupled into any of the tubes to excite an evanescent mode that couples to the fundamental cavity mode. The tubes also provide spatial and optical access for transporting atoms into the cavity, where they can be trapped while spatially overlapping the fundamental cavity mode. The piece of metal may be superconducting, allowing the resonator to be used in a cryogenic environment for quantum computing and information processing.
Abstract Millimeter-wave superconducting resonators are a useful tool for studying quantum device coherence in a new frequency domain. However, improving resonators is difficult without a robust and reliable method for coupling millimeter-wave signals to 2D structures. We develop and characterize a tapered transition structure coupling a rectangular waveguide to a planar slotline waveguide with better than 0.5 dB efficiency over 14 GHz, and use it to measure ground-shielded resonators in the W band (75–110 GHz). Having decoupled the resonators from radiative losses, we consistently achieve single-photon quality factors above 10 5 , with a two-level-system loss limit above 10 6 , and verify the effectiveness of oxide removal treatments to reduce loss. These values are 4–5 times higher than those previously reported in the W band, and much closer to typical planar microwave resonators. The improved losses demonstrated by these on-chip millimeter-wave devices shed new light on quantum decoherence in a different frequency regime, offer increased selectivity for high-frequency detectors, and enables new possibilities for hybrid quantum experiments integrating millimeter-wave frequencies.
From fundamental studies of light-matter interaction to applications in quantum networking and sensing, cavity quantum electrodynamics (QED) provides a platform-crossing toolbox to control interactions between atoms and photons. The coherence of such interactions is determined by the product of the single-pass atomic absorption and the number of photon round-trips. Reducing the cavity loss has enabled resonators supporting nearly 1-million optical roundtrips at the expense of severely limited optical material choices and increased alignment sensitivity. The single-pass absorption probability can be increased through the use of near-concentric, fiber or nanophotonic cavities, which reduce the mode waists at the expense of constrained optical access and exposure to surface fields. Here we present a new high numerical-aperture, lens-based resonator that pushes the single-atom-single-photon absorption probability per round trip close to its fundamental limit by reducing the mode size at the atom below a micron while keeping the atom mm-to-cm away from all optics. This resonator provides strong light-matter coupling in a cavity where the light circulates only ~ 10 times. We load a single 87Rb atom into such a cavity, observe strong coupling, demonstrate cavity-enhanced atom detection with imaging fidelity of 99.55(6) percent and survival probability of 99.89(4) percent in 130 microseconds, and leverage this new platform for a time-resolved exploration of cavity cooling. The resonator's loss-resilience paves the way to coupling of atoms to nonlinear and adaptive optical elements and provides a minimally invasive route to readout of defect centers. Introduction of intra-cavity imaging systems will enable the creation of cavity arrays compatible with Rydberg atom array computing technologies, vastly expanding the applicability of the cavity QED toolbox.